1. Field of the Invention
This invention relates in general to the design of a reticle for a photolithography process.
2. Description of the Related Art
Traditional lithography for semiconductor manufacture utilize chrome and/or low transmission attenuated phase shift mask (AttPSM) features on reticles (masks) to block light from transmitting to portions of a photosensitive layer on a semiconductor wafer. The chrome and/or AttPSM patterns on the reticle are generated to resemble the polygon patterns of a design layout of a circuit.
Basic lithography problems may become more severe as the need increases for smaller pitch and finer individual features for each device generation. This need for smaller dimensions causes problems related to the loss of linearity in an imaging system. Some of these problems include high mask error factor values, low dose latitude, low depth of focus, large line-end pullback with standard binary materials (e.g. chrome) or low attenuation materials (e.g. AttPSMs).
The use of higher transmission attenuated phase shift masks and xe2x80x9cchromelessxe2x80x9d phase masks (CPL) in combination with off-axis illumination is known to help reduce these problems. High transmission AttPSM reticles have features defined by a 180 degree phase transition and a partial light transmission intensity e.g. above about 16%. Chromeless reticles have features defined by a 180 degree phase transition and 100% light transmission intensity. However, with AttPSM and chromeless reticles, there are problems with the patterning of large features where undesired extra patterns may form on the wafer or no pattern at all may form on the wafer. For chromeless reticles, increasing the size of the reticle feature may cause the corresponding feature size on the wafer to decrease. This is known as negative Mask Error Factor (MEF).
A variant of a chromeless reticle has been proposed in which desired smaller features on the wafer are formed using phase shifting features on the reticle, and larger features on the wafer are formed using standard chrome features on the reticle. Another known variant is the use of so-called zebra features on the reticle to produce intermediate feature sizes on the wafer. Zebra features are mixed features with alternating segments of binary and phase shift features. Each segment in the zebra feature is near or below the resolution limit of the optical system. Therefore, the imaged zebra features behave in a manner in between a phase shift feature and a binary feature, producing better imaging than binary but without such drastic negative MEF problems of phase shift features.
However, it is clear that to obtain the benefits of high transmission AttPSM or chromeless reticles in production of devices, new techniques are needed in order to transform traditional physical layout designs (which assume all reticle features are binary) into reticle designs compatible with the use of chromeless and zebra reticle features.